Mount

The CBI mount is an elevation-azimuth design with an additional axis
to allow rotation of the array about the source vector. The elevation
range of the mount is restricted to above 40░ to reduce ground pickup
and the size and cost of the instrument. At 40░ elevation, the ground
contribution in a CBI image is ~ 3 ÁK. This could be reduced by adding
a ground shield, but the required structure is very large and wind
loading is a serious problem. Alternatively, the ground can be
``imaged'' by observing many different fields at the same declination
but over the same range of hour angle. The ground contribution can
then be subtracted from all the field images. This approach restricts
the choice of fields but avoids an expensive ground shield.

Left: a face-on view of the CBI antenna platform showing one possible configuration of the 13 antennas.Right: the CBI at elevation 50░.

The antenna platform is an irregular hexagon ~ 6.5 m in diameter and
0.6 m thick. It is a space-frame structure with 52 1-m triangular
cells, each with four antenna locations. This allows many different
array configurations, including baselines that are not simply integer
multiples of 1 m. The pointing and stiffness requirements for the CBI
antenna platform are driven by mosaicing. A visibility phase error of
1░ at wavelength 1 cm corresponds to a 1'' pointing error on a 6-m
baseline or a displacement of 28 Ám in the surface of the antenna
platform. The azimuth and platform rotation bearings in the CBI have ~
2'' axial errors and the worst-case gravitational deformation in the
platform is ~ 50 Ám. Thus, the worst-case visibility phase error on
any CBI baseline will be ~ 5░.

The CBI mount on its concrete foundation in Chile, prior
to installation of the antennas, electronics, and dome (August
1999).